I. Field of the Invention
The present invention relates generally to frequency modulated (FM) communication systems. More particularly, the present invention relates to novel and improved digital FM audio processing in a dual-mode communication system selectively operative in either FM or code division multiple access (CDMA) modes.
II. Description of the Related Art
There are presently multiple types of cellular radiotelephone systems operating. These systems include the frequency modulated (FM) advanced mobile phone system (AMPS) and two digital cellular systems: time division multiple access (TDMA and GSM), and code division multiple access (CDMA). The digital cellular systems are being implemented to handle capacity problems that AMPS is experiencing. Dual-mode CDMA/FM radiotelephones exist which are selectively operative in either FM or CDMA modes. Telecommunications Industry Association (TIA) /Electronic Industries Association (EIA) Interim Standard 95 (IS-95), entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" sets forth the requirements and standards for a dual-mode radiotelephone which is selectively operative in either FM or CDMA modes.
In an FM communication system as dictated by IS-95, the base station and the radiotelephone communicate a supervisory audio tone (SAT) during the duration of the call. The base station transmits the SAT to the radiotelephone, and the radiotelephone must detect and transpond, or return, the same SAT to the base station in order to keep the call in progress connected. Should the radiotelephone fail to detect and or transpond the proper SAT back to the base station the call would be dropped. There are three different frequencies for the SAT, 5.97 kHz, 6 kHz, or 6.03 kHz. Each base station in a given area will transmit the SAT on a different frequency. The SAT is transmitted as an analog FM signal, and so is conveniently processed in tandem with the FM voice signal.
In a typical dual-mode radiotelephone, the audio processing of FM communication signals is performed using analog signal processing techniques, and the audio processing of the digital communication signals is performed using digital signal processing techniques. Thus, the FM and CDMA audio processing paths have traditionally used separate circuits with separate components. For example, several integrated circuits are commercially available which perform analog FM audio signal processing. Likewise, several designs exist for digital signal processing of CDMA audio. However, using separate circuits for the FM and the CDMA audio processing adds cost, weight and size to the radiotelephone. What is needed is a means of performing FM audio processing digitally in order to utilize the same circuitry for both FM and CDMA audio processing.